In the WiMAX (Worldwide Interoperability for Microwave Access) Forum, the WiMAX System Profile has been established on the basis of the IEEE (Institute of Electrical and Electronic Engineers) 802.16 standard. A WiMAX radio communication system is disclosed, for example, in Patent Literature 1.
In the WiMAX radio communication system, data are transmitted and received on the basis of every sub-frame, the OFDMA (Orthogonal Frequency Division Multiple Access) system is used as a multi-access connection system, and the TDD (Time Division Duplex) system is used as a duplex system.
The OFDMA system is a system that divides a frequency domain into sub-channels and a time domain into symbols and allocates bandwidths as slots that represent the divided domain to MSs (Mobile Stations).
The TDD system is a system that switches between a DL sub-frame and a UL sub-frame on the time domain using the same frequency for a DL (Down Link) and a UL (Up Link) between a BS (Base Station) and an MS.
In this part, with reference to FIG. 1, a frame structure of the WiMAX radio communication system will be briefly described.
Referring to FIG. 1, in the frame structure of the WiMAX, DL sub-frames and UL sub-frames are switched on the time domain (TDD system). Provided between a DL subframe and a UL subframe that are adjacent to are gap times referred to as a TTG (Transmit/Receive Transition Gap) and an RTG (Receive/Transmit Transition Gap).
In the DL sub-frame and the UL sub-frame, bandwidths are allocated as slots to MSs and data are transmitted thereto using the allocated bandwidths (OFDMA system).
Provided at the beginning of the DL sub-frame is a Preamble region including a Pilot signal, followed by a MAP region and so forth that include a signal that denotes what slots of the DL sub-frame and the UL sub-frame have been allocated to each MS. They are followed by regions (DL Bursts) that are allocated to each MS as a bandwidth for which DL data are transmitted.
On the other hand, provided at the beginning of the UL sub-frame are an Ranging region and so forth that include a Ranging signal that executes Ranging for adjustment of timing, frequency, and power on the MS side. They are followed by regions (UL Bursts) allocated to each MS as a bandwidth for which UL data are transmitted.
Moreover, the WiMAX radio communication system deals with an adaptive modulation system. The adaptive modulation system is a system that adaptively changes modulation systems and code rates of DL data and UL data between the BS and each MS depending on the propagation environment of each MS. Furthermore, in the WiMAX, data transmission speed according to a combination of a modulation system and a code rate has been specified and this data transmission speed is referred to as the physical rate.
In addition, the WiMAX radio communication system deals with a QoS (Quality of Service) class. In other words, the WiMAX radio communication system reserves bandwidths for MSs that belong to a particular QoS class such as the UGS (Unsolicited Grant Service), the ERT-VR (Extended Real Time-Variable Rate Service), the RT (Real Time)-VR, or the NRT (Non Real Time)-VR.
In this part, with reference to FIG. 2, an adaptive modulation operation for an MS that belongs to a QoS class that necessitates reservation of a bandwidth in this WiMAX radio communication system will be described.
In FIG. 2, the lower illustrations represent the relationship of positions of the BS and MS#1˜MS#3 that belong to a QoS class that necessitates reservation of bandwidths, whereas the upper illustrations represent a DL sub-frame that is transmitted from the BS to the MS#1˜MS#3 in the state that the relationship of positions shown in the lower illustrations is satisfied (these conditions apply to FIG. 3 and FIG. 6).
Referring to FIG. 2, it is assumed that the MS#1˜MS#3 are located close to the BS, that is the center of the cell, in the initial state (state 1).
When an MS is located close to the center of the cell, data can be transmitted at a high physical rate. Thus, in state 1, the BS applies, for example, a 16 QAM (Quadrature Amplitude Modulation) 3/4 (the first part represents a modulation system, whereas the second part represents a code rate; these conditions apply to the description that follows) having a high physical rate as a combination of a modulation system and a code rate for the MS#1˜MS#3. In the following, an area to which the 16 QAM 3/4 is applied is referred to as the 16 QAM area.
The number of slots that necessitates reservation of a bandwidth depends on the physical rate such that the lower the physical rate is, the more the number of slots is required. In state 1, since the 16 QAM 3/4 that applies to the MS#1˜MS#3 has a high physical rate, the number of slots that necessitates reservation of bandwidths for the MS#1˜MS#3 is small. Thus, the BS can accommodate all the MS#1˜MS#3 and the free space of the bandwidth of the DL sub-frame becomes large.
Then, it is assumed that the MS#1 has exited from the 16 QAM area and has moved toward the edge direction of the cell (state 2).
Then, the BS performs the adaptive modulation for the MS#1 so as to apply, for example, a QPSK (Quadrature Phase Shift Keying) 1/2 having a low physical rate as a combination of a modulation system and a code rate for the MS#1. Hereinafter, the area to which the QPSK 1/2 applies is referred to as the QPSK area. On the other hand, since the QPSK 1/2 that applies to the MS#1 is at a low physical rate, the number of slots that necessitates reservation of a bandwidth for the MS#1 increases.
However, since the free space of the bandwidth of the DL sub-frame is large in state 1, even if the number of slots that necessitates reservation of the bandwidth for the MS#1 increases, the BS can still accommodate all the MS#1˜MS#3. However, since the MS#1 occupies the bandwidth of the DL sub-frame, it's free space becomes small.
Then, it is assumed that the MS#2 has moved from the 16 QAM area to the QPSK area (state 3).
Then, the BS performs the adaptive modulation for the MS#2 so as to apply the QPSK 1/2 having a low physical rate as a combination of a modulation system and a code rate of the MS#2.
However, since the free space of the bandwidth of the DL sub-frame is small in state 2, the BS cannot allocate a bandwidth according to the physical rate of the QPSK 1/2 to the MS#2, resulting in a drop of the physical rate of the MS#2. On the other hand, since there are no slots that can be allocated to the MS#3, the BS cannot allocate slots to the MS#3 until the next sub-frame occurs.
Thus, in the WiMAX radio communication system, to solve the foregoing problem, the BS can perform a control referred to as the Admission Control. The Admission Control is a reception control for the adaptive modulation such that the BS deals with an MS that belongs to a QoS class that necessitates reservation of a bandwidth in MSs for which the BS executes the adaptive modulation and determines whether or not the BS can accommodate MS when executing the adaptive modulation for MS. The BS does not change the physical rate of MSs for which the BS has determined that it cannot accommodate MSs.
In this part, with reference to FIG. 3, an adaptive modulation operation that involves the Admission Control for MSs that belong to a QoS class that necessitates reservation of bandwidths in the WiMAX radio communication system will be described. The relationship of positions in the lower illustrations shown in FIG. 3 is the same as that shown in FIG. 2.
Referring to FIG. 3, it is assumed that the MS#1˜MS#3 are located in the 16 QAM area in the initial state like the case shown in FIG. 2 (state 1). At this point, bandwidths have been allocated to the MS#1˜MS#3 like the case shown in FIG. 2 and the free space of the bandwidth of the DL sub-frame becomes large.
Then, it is assumed that the MS#1 has moved from the 16 QAM area to the QPSK area (state 2)
Then, the BS executes the Admission Control for the MS#1 so as to determine whether or not the BS can allocate a bandwidth according to the physical rate of the QPSK 1/2 to the MS#1 when executing the adaptive modulation for the MS#1 according to the QPSK 1/2. At this point, since the free space of the bandwidth of the DL sub-frame is large in state 1, the BS determines that it can allocate a bandwidth to the MS#1 and accommodate the MS#1. Thus, the BS accommodates all the MS#1˜MS#3 even in state 2. However, since the MS#1 occupies the bandwidth of the DL sub-frame, it's free space becomes small.
Then, it is assumed that the MS#2 has moved from the 16 QAM area to the QPSK area (state 3).
Then, the BS executes the Admission Control for the MS#2 like the case of the MS#1. However, since the free space of the bandwidth of the DL sub-frame is small in state 2, the BS determines that it cannot allocate a bandwidth to the MS#2. Thus, communication errors frequency occur in the MS#2 that cannot use an appropriate physical rate and exits from the entry of the BS. In this case, since the priority of the MS#2 is the same as that of the MS#1, unfairness take place.